Figure 1.

Breakage of a dicentric formed by chromosome fusion. (A) Segregation during one cell cycle of a dicentric formed in G1. The centromeres are in red. In anaphase, the two centromeres of each sister chromatid can migrate toward the same pole, thus propagating the dicentric intact to the next generation in the two offspring. Alternatively, the centromeres of each sister chromatid can migrate toward opposite poles, forming two anaphase bridges. (B) Fusion of the chromosome 6 right arm to another chromosome end by homologous recombination. (C) Loss of viability on galactose for cells with a conditional monocentric chromosome 6 and on glucose in response to CEN6 reactivation for cells with a conditional 6 + 7 dicentric. Fusion between chromosomes 6 and 7 deletes 1975 bp from the right end of chromosome 6 and 15,271 bp from the left end of chromosome 7. Overnight cultures of strains Lev346 (wild-type [WT]), Lev752 (conditional monocentric), Lev801, and Lev802 (two independent clones with a conditional 6 + 7 dicentric) were diluted by 10-fold serial dilutions, plated on galactose synthetic medium and glucose-rich medium, and incubated for 3 d (galactose) or 2 d (glucose) at 30°C. The ade2-1 mutation present in this background causes the cells to accumulate purine precursors when they start to exhaust the medium. These precursors give a red color to colonies. Colonies with a large fraction of sick or dead cells remain whiter. (D) Breakage of a 6 + 7 dicentric. Cells from strains Lev801 and Lev802 (6 + 7 dicentric) were grown exponentially in galactose-containing medium, synchronized in G1 with α factor, released in glucose-containing medium, and either blocked in G2/M with nocodazole (G2/M) or allowed to proceed through mitosis to be blocked in the next G1 with α factor (next G1). Chromosomes were separated by PFGE, labeled with Gel Red (left panel), and probed with a fragment from the chromosome 7 right arm (middle panel) or chromosome 6 left arm (right panel). The wild-type control was from strain Lev752 grown in glucose-containing medium. The red arrowhead marks the position of the molecules broken near CEN6.